The present invention relates to a separation control circuit for a stereophonic receiver, and more particularly to a separation control circuit for preventing deterioration of the SN (signal-to-noise) ratio of the reproduced signal by controlling the amount of separation of a stereophonic signal in proportion to the received field strength.
In a conventional separation control circuit as shown in FIG. 1, left- and right-channel signals L and R are applied to respective output terminals L.sub.o and R.sub.o through respective coupling capacitors C.sub.2 and C.sub.3. A variable impedance circuit composed of a parallel combination of transistors 20 and 21 and a series-connected capacitor C.sub.1 for mixing purposes is connected between the terminals L.sub.o and R.sub.o. An AGC voltage is applied to control the output current of a variable current source 22, and the current output of the latter is applied as a bias current to the bases of the transistors 20 and 21 so that the impedance of the variable impedance circuit is controlled thereby.
In the circuit thus arranged, the impedance Z of the variable impedance circuit changes in proportion to changes in the AGC voltage, the latter varying according to the received field strength. Because the impedance Z becomes smaller as the received strength drops, the high-frequency components of the left- and right-channel signals L and R will be reduced due to an increased amount of mixing between channels through the variable impedance circuit. Accordingly, with it so arranged that current from the current source 22 increases when the AGC voltage decreases as the field strength is reduced, lowering of the SN ratio in the reproduced signal is reduced because of a reduction in the high-frequency separation when the field strength drops.
As set forth above, separation in the high-frequency range is reduced as the impedance Z of the variable impedance approaches 0. However, because the frequency characteristics of the amplifier do not change when the amount of separation is reduced, still there can occur some degeneration of the SN ratio when the field strength is lowered. To change the frequency characteristics of the amplifier circuit together with the separation, it is necessary to provide for each channel an LPF (Low-Pass Filter) in addition to those components shown in FIG. 1. This results in a complex circuit configuration.